Holostei is a group of

ray-finned Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of skin ...

bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...

. It is divided into two major clades, the

Halecomorphi Halecomorphi is a taxon of Actinopterygii, ray-finned bony fish in the clade Neopterygii. The only extant Halecomorph species are the bowfin (''Amia calva'') and eyespot bowfin (''Amia ocellicauda''), but the group contains many extinct species i ...

, represented by the single living genus, '' Amia'' with two species, the

bowfin The ruddy bowfin (''Amia calva'') is a ray-finned fish native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being one of only two surviving species ...

s (''

Amia calva The ruddy bowfin (''Amia calva'') is a ray-finned fish native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict species, relict, being one of only two sur ...

'' and ''

Amia ocellicauda ''Amia ocellicauda'', the eyespot bowfin or emerald bowfin, is a species of Amia (fish), bowfin native to North America. Originally described by John Richardson (naturalist), John Richardson from Lake Huron in 1836, it was synonymized with ''Amia ...

''), as well as the

Ginglymodi Ginglymodi is a clade of ray-finned fish containing modern-day gars (Lepisosteidae) and their extinct relatives (including the family (biology), family Lepidotidae) in the Order (biology), order Lepisosteiformes, the extinct orders Semionotiforme ...

, the sole living representatives being the gars (Lepisosteidae), represented by seven living species in two genera (''

Atractosteus ''Atractosteus'' (from Greek ''atraktos'' (ἀτρακτὀς), 'spindle' and ''osteon'' (ὀστέον), 'bone') is a genus of gars in the family Lepisosteidae, with three extant species. It is one of two surviving gar genera alongside ''Lepisost ...

'', ''

Lepisosteus ''Lepisosteus'' (from Greek ''lepis'' (), 'scale' and ''osteon'' (), 'bone') is a genus of gars in the family Gar, Lepisosteidae. It contains four extant species, found throughout eastern and central North America. It is one of two extant gar gen ...

''). The earliest members of the clade, which are putative " semionotiforms" such as ''

Acentrophorus ''Acentrophorus'' is an extinct genus of prehistoric freshwater and marine ray-finned fish from the Roadian (Guadalupian/Middle Permian) to the Wuchiapingian (Lopingian/late Permian) of England (Marl Slate), Germany (Kupferschiefer), Italy (Val G ...

'' and '' Archaeolepidotus'', are known from the

Middle Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek ...

Late Permian Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...

and are among the earliest known neopterygians. Holostei was thought to be regarded as

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. However, a recent study provided evidence that the Holostei are the closest living relatives of the

Teleostei Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

, both within the

Neopterygii Neopterygii (from Greek νέος ''neos'' 'new' and πτέρυξ ''pteryx'' 'fin') is a subclass of ray-finned fish (Actinopterygii). Neopterygii includes the Holostei and the Teleostei, of which the latter comprise the vast majority of extant ...

. This was found from the morphology of the Holostei, for example presence of a paired

vomer The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...

. Holosteans are closer to teleosts than are the chondrosteans, the other group intermediate between teleosts and cartilaginous fish, which are regarded as (at the nearest) a sister group to the Neopterygii. The spiracles of holosteans are reduced to vestigial remnants and the bones are lightly ossified. The thick

ganoid A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...

scales of the gars are more primitive than those of the bowfin.

Characteristics

Holosteans share with other non-teleost

ray-finned fish Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of sk ...

a mixture of characteristics of

teleosts Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all extant species of fish. The Teleostei, which i ...

and

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s. In comparison with the other group of non-teleost ray-finned fish, the chondrosteans, the holosteans are closer to the teleosts and further from sharks: the pair of spiracles found in sharks and chondrosteans is reduced in holosteans to a remnant structure: in gars, the spiracles do not even open to the outside; the skeleton is lightly

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

: a thin layer of bone covers a mostly

cartilaginous Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

skeleton in the bowfins. In gars, the tail is still

heterocercal Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

but less so than in the chondrosteans. Bowfins have many-rayed dorsal fins and can breathe air like the

bichir Bichirs and the reedfish comprise Polypteridae , a family (biology), family of archaic Actinopterygii, ray-finned fishes and the only family in the order (biology), order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The D ...

s. In the holosteans a primary pulmonoid (respiratory)

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ (anatomy), organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift ...

is still present, a trait that was independently lost in both chondrostei and teleostei, the only other two lineages of fish with a swim bladder (in some teleosts the swim bladder have since evolved to become secondarily respiratory again). The gars have thick

ganoid scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...

s typical of

sturgeon Sturgeon (from Old English ultimately from Proto-Indo-European language, Proto-Indo-European *''str̥(Hx)yón''-) is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the ...

s whereas the bowfin has thin bony scales like the teleosts. The gars are therefore in this regard considered more primitive than the bowfin. The name ''Holostei'' derives from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

words ''holos'', meaning whole, and ''osteon'', meaning bone: a reference to their bony skeletons.

Systematics of Neopterygii

The evolutionary relationships of gars, bowfin and teleosts were a matter of debate. There are two competing

hypotheses A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific method, scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educ ...

on the systematics of neopterygians:

Halecostomi hypothesis

The Halecostomi hypothesis proposes

(

and its fossil relatives) as the

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

, the major group of living neopterygians, rendering the Holostei

Holostei hypothesis

The Holostei hypothesis, where the gars and bowfin form the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Holostei as the

to Teleostei, is better supported than the Halecostomi hypothesis, rendering the latter paraphyletic. It proposes Halecomorphi as the sister group of

, the group which includes living gars (

Lepisosteiformes Lepisosteiformes is an order of ray-finned fish and the only living members of the clade Ginglymodi. Its only extant representatives are the gar (family Lepisosteidae), and it is defined as all members of Ginglymodi that are more closely rela ...

) and their fossil relatives. It is estimated that the

last common ancestor A most recent common ancestor (MRCA), also known as a last common ancestor (LCA), is the most recent individual from which all organisms of a set are inferred to have descended. The most recent common ancestor of a higher taxon is generally assu ...

of gars and bowfin lived at least 250 million years ago. Ginglymodi comprises three

orders Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * H ...

Semionotiformes Semionotiformes is an order of ray-finned fish known from the Middle Triassic (Anisian) to the Late Cretaceous (Maastrichtian). Their closest living relatives are gars (Lepisosteidae), with both groups belonging to the clade Ginglymodi within th ...

and

Kyphosichthyiformes Ginglymodi is a clade of ray-finned fish containing modern-day gars (Lepisosteidae) and their extinct relatives (including the family Lepidotidae) in the order Lepisosteiformes, the extinct orders Semionotiformes and Kyphosichthyiformes, and v ...

. Lepisosteiformes includes 1

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

, 2

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

, and 7

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

that are commonly referred to as gars. Semionotiformes and Kyphosichthyiformes are extinct orders. Halecomorphi contains the orders

Parasemionotiformes Parasemionotiformes is an extinct order of neopterygian ray-finned fish that existed globally during the Triassic period. It comprises the families Parasemionotidae and Promecosominidae. Many of the included genera are monotypic and most specie ...

, Panxianichthyiformes, Ionoscopiformes, and

Amiiformes The Amiiformes order (biology), order of fish has only two extant taxa, extant species, the bowfins: ''Amia calva'' and ''Amia ocellicauda'', the latter recognized as a separate species in 2022. These Amiiformes are found in the freshwater syste ...

. In addition to many

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

, Amiiformes includes only 1

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

species that is commonly referred to as the bowfin. Parasemionotiformes, Panxianichthyiformes, and Ionoscopiformes have no living members. Gars and bowfins are found in North America and in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

ecosystems. The differences in each can be spotted very easily from just looking at the fishes. The gars have elongated jaws with fanlike teeth, only 3 branchiostegal rays, and a small dorsal fin. Meanwhile the bowfins have a terminal mouth, 10–13 flattened branchiostegal rays, and a long dorsal fin.

Phylogeny of bony fishes

The

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

shows the relationships of holosteans to other living groups of

(Osteichthyes), the great majority of which are

teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

s, and to the terrestrial vertebrates (tetrapods) that evolved from a related group of

lobe-finned fish Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates ar ...

. Approximate dates are from Near et al. (2012).

Notes

References

External links

Holostei on The University of Liverpool website
{{Taxonbar, from=Q846164 Neopterygii Extant Jurassic first appearances